Bump structures for multi-chip packaging

ABSTRACT

A multi-chip package includes a substrate having a plurality of first bump structures. A pitch between first bump structures of the plurality of first bump structures is uniform across a surface of the substrate. The multi-chip package includes a first chip bonded to the substrate and a second chip bonded to the substrate. The first chip includes a plurality of second bump structures, and the plurality of second bump structures are bonded to a first set of first bump structures of the plurality of first bump structures. The second chip includes a plurality of third bump structures, and the plurality of third bump structures are bonded to a second set of first bump structures of the plurality of first bump structures. A pitch between second bump structures of the plurality of second bump structures is different from a pitch between third bump structures of the plurality of third bump structures.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No. 13/427,753, filed Mar. 22, 2012, which claims priority of U.S. Provisional Patent Application No. 61/564,594 filed Nov. 29, 2011, all of which are incorporated herein by reference in their entireties.

BACKGROUND

The fabrication of modern circuits involves several steps. Integrated circuits are first fabricated on a semiconductor wafer, which contains multiple duplicated semiconductor chips, each comprising integrated circuits. The semiconductor chips are then sawed from the wafer and packaged. The packaging processes have two main purposes: to protect delicate semiconductor chips, and to connect interior integrated circuits to exterior connections.

In packaging integrated circuit (IC) chips, solder joining is one of the commonly used methods for bonding IC chips to package substrates, which may or may not include integrated circuits and/or other passive components. In packaging processes, a semiconductor die (or chip) may be mounted on a package substrate using flip-chip bonding. The package substrate may be an interposer that includes metal connections for routing electrical signals between opposite sides. Other types of substrates may also be used. The die may be bonded to the substrate through direct metal bonding, solder bonding, or the like. There are many challenges in chip packaging.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1A and 1B show cross sectional views of a process sequence for forming bump structures between an integrated circuit (IC) die (or chip) and a substrate, in accordance with some embodiments.

FIGS. 1C and 1D show cross-sectional views of two bump structures, in accordance with some embodiments.

FIG. 2A shows a top view of a multi-chip package with a number of chips bonded to a substrate, in accordance with some embodiments.

FIG. 2B shows a cross-sectional view of a portion of the multi-chip package of FIG. 2A cut along line P-P, in accordance with some embodiments.

FIGS. 2C and 2D show cross-sectional views of two bump structures, in accordance with some embodiments.

FIGS. 3A and 3B show cross sectional views of a process sequence for forming bump structures between a chip and a substrate, in accordance with some embodiments.

FIGS. 3C and 3D show cross sectional views of a process sequence for forming bump structures between a chip and a substrate, in accordance with some embodiments.

FIG. 3E shows top views of different numbers and arrangements of micro-bumps for bonding to a larger flip-chip bump, in accordance with some embodiments.

FIG. 4 shows a process flow of forming a multi-chip package, in accordance with some embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative, and do not limit the scope of the disclosure.

FIG. 1A shows a cross sectional view of an integrated circuit (IC) die (or chip) 120 and a substrate 125 after bumps 121 and 126 are formed respectively, in accordance with some embodiments. Bumps 121 and 126 are connected to metal pads 128 _(C) and 128 _(S) via under bump metallurgy (UBM) layers 145 _(C) and 145 _(S) respectively, as shown in FIG. 1A. Bumps 121 are aligned with bumps 126 for bonding. The width of bumps 121 is W. In some embodiments, the width of bumps 126 is about the same as the width of bumps 121. Substrate 125 may be a semiconductor wafer, or a portion of a wafer. Substrate 125 may include silicon, gallium arsenide, silicon on insulator (“SOI”) or other similar materials. Substrate 125 may also include passive devices such as resistors, capacitors, inductors and the like, or active devices such as transistors. Substrate 125 may be an interposer and may further include through substrate vias (TSVs) 135, as shown in FIG. 1A. In addition, the substrate 125 may also be of other materials in alternative embodiments. For example, multiple layer circuit boards may be used. Substrate 125 may also include bismaleimide triazine (BT) resin, FR-4 (a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant), ceramic, glass, plastic, tape, film, or other supporting materials that may carry the conductive pads or lands needed to receive the connector terminals 115 for the flip-chip IC die 120.

FIG. 1B shows a cross-sectional view of chip 120 bonded to substrate 125 to form package 122, in accordance with some embodiments. Bumps 121 and 126 are joined together by a solder layer 123, which is formed of solder from bumps 121 and 126, to form bump structures 127. The bump structures 127 in FIG. 1B have a pitch P and a spacing (or distance) S between bumps 127.

For advanced packaging of IC dies with many function circuitries, the sizes of bumps 121 and 126 are relatively small to enable more bumps to connect to an input/output (I/O) of chip 120. In some embodiments, the widths of bumps 121 and 126 are in a range from about 5 μm to about 40 μm, in accordance with some embodiments. Such bumps may also be called micro-bumps. In some other embodiments, the widths of bumps 121 and 126 are smaller and range from about 2 μm to about 10 μm. Micro-bumps may include copper posts and may be called copper post (or pillar) bumps. The pitch P of bumps (micro-bumps) 121 and 126 are in a range from about 10 μm to about 60 μm, in accordance with some embodiments. The spacing S of bumps (micro-bumps) 121 and 126 are in a range from about 5 μm to about 30 μm, in accordance with some embodiments. In some other embodiments, when the widths of bumps 121 and 126 ranges from about 2 μm to about 10 μm, the spacing S of bumps (micro-bumps) 121 and 126 ranges from about 1.5 μm to about 10 μm.

FIG. 1C shows a bump structure 100 with a substrate 110, in accordance with some embodiments. Substrate 110 may be a semiconductor substrate, such as a bulk silicon substrate, although it may include other semiconductor materials, such as group III, group IV, and/or group V elements. Substrate 110 may include silicon, gallium arsenide, silicon on insulator (“SOI”) or other similar materials. Semiconductor devices 114, such as transistors, may be formed at the surface of substrate 110. Substrate 110 may also include passive devices such as resistors, capacitors, inductors and the like, or active devices such as transistors. Substrate 100 may, in an exemplary embodiment, include additional integrated circuits. Substrate 110 may be an interposer. In addition, the substrate 110 may also be of other materials in alternative embodiments. For example, multiple layer circuit boards may be used. Substrate 110 may also include bismaleimide triazine (BT) resin, FR-4 (a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant), ceramic, glass, plastic, tape, film, or other supporting materials.

An interconnect structure 112, which includes metal lines and vias (not shown) formed therein and connected to semiconductor devices 114, is formed over substrate 110. The metal lines and vias may be formed of copper or copper alloys, and may be formed using the well-known damascene processes. Interconnect structure 112 may include commonly known inter-layer dielectrics (ILDs) and inter-metal dielectrics (IMDs).

A metal pad 128 is formed over interconnect structure 112. Metal pad 128 may comprise aluminum, and hence may also be referred to as aluminum pad 128, although it may also be formed of, or include, other materials, such as copper, silver, gold, nickel, tungsten, alloys thereof, and/or multi-layers thereof. Metal pad 128 may be electrically connected to semiconductor devices 114, for example, through underlying interconnection structure 112. The metal pad 128 may be a top metal layer or a redistribution layer (RDL). In some embodiments, a passivation layer 130 is formed to cover edge portions of metal pad 128. The passivation layer 130 may be formed of polyimide or other suitable dielectric materials. Additional passivation layers may be formed over interconnect structure 112 and at the same level, or over, metal pad 128. The additional passivation layers may be formed of materials such as silicon oxide, silicon nitride, un-doped silicate glass (USG), polyimide, and/or multi-layers thereof.

The bump structure 100 includes a diffusion barrier layer 140 and a thin seed layer 142, in accordance with some embodiments. Diffusion barrier layer 140 may be a titanium layer, a titanium nitride layer, a tantalum layer, or a tantalum nitride layer. The materials of seed layer 142 may include copper or copper alloys, and hence is referred to as copper seed layer 142 hereinafter. However, other metals, such as silver, gold, aluminum, and combinations thereof, may also be included. The combined diffusion barrier layer 140 and copper seed layer 142 may also be referred to as an under bump metallurgy (UBM) layer 145.

In some embodiments, bump structure 100 also includes a copper layer 150, a metal layer 152, and a solder layer 160. The copper layer 150, the metal layer 152, and the solder layer 160 are formed by plating with a photo mask defining the openings, in accordance with some embodiments. In some embodiments, metal layer 152 is a nickel-containing layer comprising, for example, a nickel layer or a nickel alloy layer by plating. Metal layer 152 prevents the formation of an inter-metallic compound (IMC) between copper and solder. Solder layer 160 may be a lead-free pre-solder layer formed of, for example, SnAg, or a solder material, including alloys of tin, lead, silver, copper, nickel, bismuth, or combinations thereof. In FIG. 1A, the solder layer 160 is rounded as a result of reflow. In some embodiments, bump structure 100 does not include solder layer 160. In some embodiments, bump structure 100 does not include solder layer 160 and metal layer 152.

When the thickness of copper layer 150 is larger than the thickness of solder layer 160, the bump structure is referred to as a copper post (or pillar) bump. For advanced chip packaging, the bump pitch and bump width are reduced. Copper post bump enables reduction of bump pitch and width. The embodiment shown in FIG. 1A is merely an example; other embodiments of bumps are also possible. Further details of bump formation process may be found in U.S. patent application Ser. No. 12/842,617, filed on Jul. 23, 2010 and entitled “Preventing UBM Oxidation in Bump Formation Processes,” and U.S. patent application Ser. No. 12/846,353, filed on Jul. 29, 2010 and entitled “Mechanisms for Forming Copper Pillar Bumps,” both of which are incorporated herein in their entireties.

FIG. 1D shows a bump structure 150, in accordance with some other embodiments. Bump structure 150 has many features similar to bump structure 100. The same numbering is used for similar layers or structures. Bump structure 150 does not have solder layer 160. In addition, the metal layer 152″ is formed to cap an entire surface of copper layer 150. Copper layer 150 formed after the UBM layer 145 and extending from the boundary of copper layer 150 has been removed.

With the increased popularity of handheld electronic devices, memory chips are packaged with logic chip(s) to improve the package form factor. A chip package with more than one chip is called multi-chip package. Some chips, such as memory chips, have lower counts of input/output (I/O) connections. Such chips are manufactured with larger bumps, due to the relatively lower number of I/O connections needed. In addition, larger bumps are easier to make and can be made by less advanced processing technologies. FIG. 2A shows a top view of a multi-chip package 200 with a number of chips 201-205 bonded to a substrate 210, in accordance with some embodiments. Substrate 210 has bumps to bond with bumps on chips 201-205. Although in some embodiments the bumps on substrate 210 have different sizes, the manufacturing process is more complicated and more expensive as the number of different sized bumps increases. As a result, in some embodiments, the bumps on substrate 210 have about the same sizes.

Chips 201-204 are chips with low numbers of I/O connections (bumps), such as memory chips compared to chip 205 with higher number of bumps. For example, chip 205 could be a logic chip, which needs a large number of I/O connections to achieve its functions. As a result, bumps with fine pitches and sizes, such as micro-bumps, are used for external connections. In contrast, memory chips 201-204 do not need such bumps, since the number of bumps needed are much lower. It is also possible to make the bump sizes and pitches for memory chips 201-204 to be the same as those for logic chip 205; however, not every memory manufacturer has the capability or capacity to make smaller bumps, such as micro-bumps. It is a challenge to bond chips with different bump sizes on a single substrate.

FIG. 2B shows a cross-sectional view of a portion of multi-chip package 200 of FIG. 2A cut along line P-P, in accordance with some embodiments. FIG. 2B shows that chip 201 mounted on substrate 210 with larger bump structures 221 than the bump structures 222 for chip 205. Although FIG. 2B shows that chips 201 and 205 are at the same height after bonding, this is not a requirement. Chips 201 and 205 could be at different heights after bonding. Bump structures 221 and 222 are represented by round shapes in FIG. 2B for simplicity. An exemplary bump structure 222 is bump structure 127, whose formation process has been described above and shown in FIGS. 1A and 1B. Details of how to form bump structures 221 are described below.

FIG. 2C shows a cross-sectional view of a flip-chip bump 100* for chip 201, in accordance with some embodiments. The various layers in flip-chip bump 100* are similar to those of micro-bump 100 (FIG. 1C) described above. The width of bump 100* is larger than bump 100, which is a micro-bump in accordance with some embodiments. In some embodiments, the width of bump 100* is greater than about 40 μm and equal to or less than about 120 μm. Bumps 100* may also be called C4 bump. C4 stands for controlled collapse chip connection. In addition, the ratio of the thickness of copper layer 150* to the thickness of solder layer 160* of bump 100* is different from the ratio for bump 100. Bump 100* has a solder layer 160* thicker than the copper layer 150* and is not a copper post bump. In contrast, micro-bump 100 is a copper post bump with copper layer 150 being thicker than solder layer 160. In some embodiments, the thickness of copper layer 150* of bump 100* is in a range from about 5 μm to about 50 μm. The thickness of solder layer 160* is in a range from about 15 μm to about 60 μm, in accordance with some embodiments.

FIG. 2D shows a cross-sectional view of a flip-chip bump 100′ for chip 201, in accordance with some embodiments. The various layers in flip-chip bump 100′ are similar to those of bump 100* described above. However, bump 100′ does not have copper layer 150* and metal layer 152* of FIG. 2C. The solder layer 160′ is directly deposited on UBM layer 145′. The solder layer 160′ is rounded due to reflow. In some embodiments, the UBM layer 145′ does not include the copper seed layer 142′. The range of width of flip-chip 100′ is similar to flip-chip 100*. In some embodiments, the thickness of solder layer 160′ is in a range from about 15 μm to about 120 μm.

FIG. 3A shows a cross-sectional view of chip 201 _(A) with a bump 231 _(A) being placed above substrate 210 _(A) with micro-bumps 241 _(A) and 242 _(A), in accordance with some embodiments. Bump 231 _(A) has a structure described in FIG. 2C. The structures of micro-bumps 241 _(A) and 242 _(A) have been described in FIG. 1C. In some embodiments, micro-bumps 241 _(A) and 242 _(A) do not have solder layer 160 _(A) and metal layer 152 _(A). Chip 201 _(A) and substrate 210 _(A) are then pressed together to allow bump 231 _(A) to come in contact with micro-bumps 241 _(A) and 242 _(A). Afterwards, the solder layers of the bumps 231 _(A), 241 _(A), and 242 _(A) are reflowed to form a single layer (or entity) 233 _(A), which is part of bump structure 245 _(A) as shown in FIG. 3B, in accordance with some embodiments. By using more than one micro-bump, bumps 241 _(A) and 242 _(A), of substrate 210 _(A) to contract flip-chip bump 231 _(A) of chip 201 _(A), the bump structure 245 _(A) is stronger than a bump structure involving only one single micro-bump (with only bump 241 _(A) or bump 242 _(A)). In addition, micro-bumps 241 _(A) and 242 _(A) can share the burden of carrying current to or from bump 231 _(A).

FIG. 3C shows a cross-sectional view of chip 201 _(B) with a bump 231 _(B) being placed above substrate 210 _(B) with micro-bumps 241 _(B) and 242 _(B), in accordance with some embodiments. Bump 231 _(B) has a structure described in FIG. 2D. The structures of micro-bumps 241 _(B) and 242 _(B) have been described in FIG. 1D. In some embodiments, 241 _(B) and 242 _(B) do not have metal layer 152 _(B). Chip 201 _(E) and substrate 210 _(B) are then pressed together to allow bumps 231 _(B) to come in contact with micro-bumps 241 _(B) and 242 _(B). Afterwards, the solder layer 160 ^(B) of the bump 231 _(B) is reflowed to become layer 233 _(B), which surrounds micro-bumps 241 _(B) and 242 _(B), and forms bump structure 245 _(B), as shown in FIG. 3D, in accordance with some embodiments. The bumps on chips and substrates and bump structures formed described in FIGS. 3A-3D are merely examples. Other types or bumps and combinations of bumps on chips and substrates may also be used to formed different variation of bump structures.

The bump structures 245A and 245B described above in FIGS. 3B and 3D involved only two micro-bumps in each structure. Alternatively, more than two micro-bumps may be used to connect with a flip-chip bump. FIG. 3E shows exemplary top views of different numbers and arrangements of micro-bumps for bonding to a larger flip-chip bump. FIG. 3E (I) shows 3 micro-bumps spaced evenly. FIG. 3E (II) shows 4 micro-bumps spaced evenly. FIGS. 3E (III) and (IV) show two different arrangements of 5 micro-bumps for bonding with a flip-chip bump. FIG. 3E (V) shows 6 micro-bumps spaced evenly. However, un-evenly spaced micro-bumps may also be used. The numbers and arrangements of micro-bumps shown in FIG. 3E area merely examples. Additional numbers and/or different arrangements of micro-bumps may also be used.

Although chips 201-204 of FIGS. 2A and 2B and chips 201 _(A) and 201 _(B) of FIGS. 3A-3D are described above as memory chips, chips 201-204 and chips 201 _(A) and 201 _(E) could be any chips with flip-chip bumps larger than the bumps on chip 205. The mechanism of forming a bump structure by bonding a large bump with two or more smaller bumps can be applied to various packaged devices. The smaller bumps on the substrate 210, 210 _(A), and/or 210 _(B) do not need to be micro-bumps. They just need to be smaller than bumps on chips 201-204, 201 _(A) and 210 _(B). The mechanisms can apply for bonding bumps on chips with bumps having different sizes on substrates. For example, the larger bump could be a micro-bump with a width in a range from about 10 μm to about 40 μm and the smaller bumps could be bumps smaller than micro-bumps, with a width in a range from about 2 μm to about 10 μm. Two or more such bumps that are smaller than micro-bumps may be bonded to a micro-bump in mechanisms described above.

FIG. 4 shows a process flow 400 of forming a multi-chip package, in accordance with some embodiments. At operation 401, two chips with different bump sizes are provided. Each of the bumps on a chip are about the same size. One chip has a bump size much larger than a bump size of the other chip, such as equal to or greater than about 1.5 times. At operation 402, a substrate for bonding with the two chips is provided. The substrate has bumps with sizes about the same as the chip with smaller bumps. In some embodiments, the pitch(es) of the bumps on the substrate are about the same as the pitch(es) on the chip with smaller bumps. The order of operations 401 and 402 can be reversed. At operation 403, the chips are placed on the substrate with the bumps on the chips aligned above the bumps on the substrate. Each of the bumps on the chip with larger bumps is disposed above more than one bumps on the substrate. Afterwards, at operation 404, the bumps are pressed together and the solder between the aligned bumps are reflowed to form bump structures between the chips and the substrate. A multi-chip package is thus formed. Additional processing may be performed to complete the packaging process. For example, underfill may be formed to fill the space between the chips and the substrate.

The mechanisms for forming a multi-chip package described above enable chips with different bump sizes being packaged to a common substrate. A chip with larger bumps can be bonded with two or more smaller bumps on a substrate. Conversely, two or more small bumps on a chip may be bonded with a large bump on a substrate. By allowing bumps with different sizes to be bonded together, chips with different bump sizes can be packaged together to form a multi-chip package.

One aspect of this description relates to a multi-chip package. The multi-chip package includes a substrate having a plurality of first bump structures. A pitch between adjacent first bump structures of the plurality of first bump structures is uniform across a surface of the substrate. The multi-chip package further includes a first chip bonded to the substrate. The first chip includes a plurality of second bump structures, and the plurality of second bump structures are bonded to a first set of first bump structures of the plurality of first bump structures. The multi-chip package further includes a second chip bonded to the substrate. The second chip includes a plurality of third bump structures, and the plurality of third bump structures are bonded to a second set of first bump structures of the plurality of first bump structures. A pitch between adjacent second bump structures of the plurality of second bump structures is different from a pitch between adjacent third bump structures of the plurality of third bump structures.

Another aspect of this description relates to a multi-chip package. The multi-package chip includes a substrate having a plurality of first bump structures. The multi-package chip further includes a first chip bonded to the substrate. The first chip includes a plurality of second bump structures, and at least one second bump structure of the plurality of second bump structures covers an entirety of each sidewall of at least two first bump structures of the plurality of first bump structures. The multi-package chip further includes a second chip bonded to the substrate. The second chip includes a plurality of third bump structures, and the plurality of third bump structures are bonded to a set of first bump structures of the plurality of first bump structures.

Still another aspect of this description relates to a method of forming a multi-chip package. The method includes bonding a first chip to a substrate, wherein the substrate has a plurality of first bump structures. The first chip includes a plurality of second bump structures, and bonding the first chip to the substrate comprises covering an entirety of each sidewall of at least two first bump structures of the plurality of first bump structures with a second bump structure of the plurality of second bump structures. The method further includes bonding a second chip to the substrate. The second chip includes a plurality of third bump structures, and bonding the second chip to the substrate includes bonding the plurality of third bump structures to a set of first bump structures of the plurality of first bump structures.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. A multi-chip package comprising: a substrate having a plurality of first bump structures, wherein a pitch between adjacent first bump structures of the plurality of first bump structures is uniform across a surface of the substrate; a first chip bonded to the substrate, wherein the first chip comprises a plurality of second bump structures, and the plurality of second bump structures are bonded to a first set of first bump structures of the plurality of first bump structures; and a second chip bonded to the substrate, wherein the second chip comprises a plurality of third bump structures, and the plurality of third bump structures are bonded to a second set of first bump structures of the plurality of first bump structures, wherein a pitch between adjacent second bump structures of the plurality of second bump structures is different from a pitch between adjacent third bump structures of the plurality of third bump structures.
 2. The multi-chip package of claim 1, wherein at least one third bump structure of the plurality of third bump structures covers an entire sidewall of each first bump structure of the second set of first bump structures.
 3. The multi-chip package of claim 1, wherein first bump structures of the second set of first bump structures are arranged in a triangular pattern, a quadrilateral pattern, a pentagonal pattern, a hexagonal pattern, or a quincunx pattern.
 4. The multi-chip package of claim 1, wherein at least one third bump structure of the plurality of third bump structures comprises: a diffusion barrier layer; a copper layer over the diffusion barrier layer; a metal layer over the copper layer; and a solder layer over the metal layer.
 5. The multi-chip package of claim 4, wherein the metal layer comprises nickel.
 6. The multi-chip package of claim 4, wherein a thickness of the copper layer ranges from 5 microns (μm) to 50 μm.
 7. The multi-chip package of claim 4, wherein a thickness of the solder layer ranges from about 15 μm to 60 μm.
 8. The multi-chip package of claim 4, wherein a thickness of the solder layer is greater than a thickness of the copper layer.
 9. The multi-chip package of claim 1, wherein each first bump structure of the plurality of first bump structures is a copper post, and each third bump structure of the plurality of third bump structures is a solder bump.
 10. A multi-chip package comprising: a substrate having a plurality of first bump structures; a first chip bonded to the substrate, wherein the first chip comprises a plurality of second bump structures, and at least one second bump structure of the plurality of second bump structures covers an entirety of each sidewall of at least two first bump structures of the plurality of first bump structures; and a second chip bonded to the substrate, wherein the second chip comprises a plurality of third bump structures, and the plurality of third bump structures are bonded to a set of first bump structures of the plurality of first bump structures.
 11. The multi-chip package of claim 10, wherein at least one second bump structure of the plurality of second bump structures comprises: an under bump metallurgy (UBM) layer; and a solder layer in direct contact with the UBM layer.
 12. The multi-chip package of claim 10, wherein at least one first bump structure of the plurality of first bump structures comprises: a diffusion barrier layer; a copper layer over the diffusion barrier layer; and a nickel-containing layer over the copper layer.
 13. The multi-chip package of claim 12, wherein the nickel-containing layer covers sidewalls of the copper layer.
 14. The multi-chip package of claim 12, wherein the at least one first bump structure of the plurality of first bump structures further comprises a solder layer over the nickel-containing layer.
 15. The multi-chip package of claim 14, wherein a thickness of the copper layer is greater than a thickness of the solder layer.
 16. A method of forming a multi-chip package, the method comprising: bonding a first chip to a substrate, wherein the substrate has a plurality of first bump structures, wherein the first chip comprises a plurality of second bump structures, and bonding the first chip to the substrate comprises covering an entirety of each sidewall of at least two first bump structures of the plurality of first bump structures with a second bump structure of the plurality of second bump structures; and bonding a second chip to the substrate, wherein the second chip comprises a plurality of third bump structures, and bonding the second chip to the substrate comprises bonding the plurality of third bump structures to a set of first bump structures of the plurality of first bump structures.
 17. The method of claim 16, wherein bonding the first chip to the substrate comprises covering the entirety of each sidewall of the at least two first bump structures arranged in a triangular pattern, a quadrilateral pattern, a pentagonal pattern, a hexagonal pattern, or a quincunx pattern.
 18. The method of claim 16, wherein bonding the first chip to the substrate comprises covering the entirety of each sidewall of the at least two first bump structures, wherein each first bump structure of the plurality of first bump structures is a copper post, and each second bump structure of the plurality of second bump structures is a solder bump.
 19. The method of claim 16, wherein bonding the plurality of third bump structures to the set of first bump structures comprises bonding the plurality of third bump structures having a pitch different from a pitch of the second plurality of bump structures.
 20. The method of claim 16, wherein bonding the first chip to the substrate comprises bonding the first chip to the substrate having the plurality of first bump structures having a uniform pitch across an entire surface of the substrate. 